The present invention relates to a tripod type constant-velocity joint through which a driving shaft and a driven shaft are coupled together so as to transmit torque from the driving shaft to the driven shaft, and an image forming device including such a tripod type constant-velocity joint.
A constant-velocity joint is typically used to transmit torque from a vehicle drive shaft to an axle.
Through such a constant-velocity joint, torque can be transmitted from a driving shaft to a driven shaft such that the driven shaft is rotated at a constant speed even if the angle between the driving shaft and the driven shaft varies. Thus, constant-velocity joints are used in various industrial machines other than motor vehicles, too.
There are known two types of constant-velocity joints, i.e. fixed type constant-velocity joints, which only allow the driving and driven shafts to incline relative to each other, and sliding type constant-velocity joints, which allow the driving and driven shafts not only to incline but to axially slide relative to each other. A sliding type constant-velocity joint is disclosed in JP patent publication 3-1528 (examined).
The constant-velocity joint disclosed in this publication, known as a tripod type constant-velocity joint, comprises an outer ring having three axial track grooves formed in the inner surface thereof so as to be circumferentially spaced apart from each other at equal angular intervals of 120 degrees, and a tripod member inserted in the outer ring so that its three radial trunnions are received in the respective track grooves. A spherical roller is rotatably and axially slidably mounted on each trunnion so as to roll along the corresponding track groove. Torque is thus transmitted between the outer ring and the tripod member through the spherical rollers.
In JP patent publication 5-341589 (unexamined), an image forming device is disclosed which includes a photoconductor drum, a motor and a universal joint through which the shaft of the photoconductor drum and the rotary shaft of the motor are coupled together. The light exposure position and the image transfer position are arranged diametrically opposed to each other on the photoconductor drum so as to cancel any circumferential shrinkage and expansion of images due to fluctuations in the rotational speed of the photoconductor drum by shrinkage and expansion of images at light exposure position and the image transfer position so that the transferred images will be substantially free of shrinkage and expansion.
The tripod type constant-velocity joint disclosed in JP patent publication 3-1528 has its outer ring, tripod member and spherical rollers all made of a metal such as steel. Thus, although such a joint is high in strength, it is heavy, needs lubrication with grease, and tends to produce loud noise. Its use is thus limited. For example, such a joint is not suitable for use in office machines, audio devices, medical instruments, food producing machines, household appliances, etc.
In order to use such a constant-velocity joint in a food processing machine, grease has to be sealed completely in order to prevent contamination of foods with grease. It is therefore necessary to provide the joint with means for preventing leakage of grease. This increases the number of parts of the joint and thus its size, and pushes up its cost, too.
In the image forming device disclosed in JP patent publication 5-341589, since the universal joint is the non-constant-velocity type, the photoconductor drum cannot be rotated at a constant speed. Although this publication proposes to arrange the exposure position and the transfer position diametrically opposite to each other, it is actually difficult to arrange the above two positions exactly diametrically opposite to each other due to assembling errors and/or inclination of the photoconductor drum. Thus, it will be difficult to completely cancel any shrinkage and expansion of the latent image with the shrinkage and expansion of the toner image. The ultimately obtained image thus tends to be low in quality.
An object of the present invention is to provide a tripod type constant-velocity joint which can be easily assembled and disassembled, which needs no lubrication with grease, which is lightweight, compact in size and quiet in operation, and which is less limited in utility.
Another object of the present invention is to provide an image forming device which includes a means for rotating the photoconductor drum always at a constant speed, and of which the maintenance is easy.